Fire-resisting structure



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Patented Dec. 30, 1941 FIRE-RESISTING STRUCTURE Victor Lefebure andArthur Henryv Douglas,

London, England, assignors to Imperial Chemical Industries Limited, acorporation of Great Britain Application April 21, 1939, Serial No.269,287

In Great Britain April 28, 1938 8 Claims.

It is well known that methods of protection are employed to enablecolumns or beams in structures to retain their strength in a fire bysurrounding them with materials which resist the spread of heat from theexposed faces to the structural members, say steel columns. A commonmethod is to build a column of concrete round the structural member, bythe usual method of pouring concrete with the support of shuttering.

Attempts have been made to employ swifter, lower cost and drier methodsby erecting narrow block walls round the structural member, thusattempting to use the air-space between such wall and the member as acontribution to the thermal resistance. Such methods are deficient owingto the tendency for the protecting walls, and notably the mortar, tocrack, thus allowing ame and hot gases to penetrate beyond the wall andto Contact with the member.

This invention has as an object to devise a new method of buildingfire-proof structures. A further object is to devise a new method ofprotecting columns and beams from fire. A still further object is toprovide new fire-proof structures. Further objects will appearhereinafter. 'Ihese objects are accomplished by thel followinginvention, according to which a fire-resisting casing for beams andcolumns in structures is provided which comprises an outer casing of setcalcium sulphate plaster (gypsum) of substantial thickness, preferablyat least 3A inch, firmly bonded over the whole of its inner surface to abacking layer of strong fibrous sheet material, other than textilefabric, which forms an inner casing, or a part thereof, and which isseparately erected around the member to Abe protected in such a mannerthat air spaces of substantial 'size are formed between portions of theinner casing and portions of the said member.

A satisfactory method of building the inner casing is to use a wallboard or plaster board consisting of a core of calcium sulphate betweencoverings or liners of paper, cardboard, pulp board, or the like whichare rmly bonded to the core. This can be shaped to fit round the member,or simply employed as long strips of board. A preferred form consists oftwo U-shaped sections. 'Ihese strips or sections are tied to or aroundthe member by means of metal wire or metal ribbon, or other suitableforms of easy attachment. Alternatively, they can be stuck to the memberby means of a nre-proof adhesive such as a composition of Portlandcement, an

This method has the advantage of protect-ing the metal againstcorrosion. Metal VclipsA can be employed to hold the strips or sectionsin position, and the corners can be strengthened by means of strips oflinen, cotton Yor other suitable textile material applied with anadhesive. Thus air spaces of substantial size are formed betweenportions of the inner casing and portions of said member.

Blocks may now be erected around the structure using a bonding materialsuch as plaster, or a fire-proof adhesive such 'as that mentioned above.We prefer to employ metal strip or similar reinforcement in the bondingmaterial. As the blocks are erected they are not only bonded together,but also bonded to the inner casing.

A simple and satisfactory method of erecting the inner casing is to takea sheet of plaster board and score it in lines along the length ofalkaline silicate and a retarder such as glue.

the board, thus marking oli rectangles corresponding with the dimensionsof the section to be protected. 'Ihe scoring is through the liner and asdeep into the core as possible without harming the bottom liner. Theboard can then very easily be broken along the linesv of the scoringwithout tearing the back liner. It can then be folded round the memberto be protect-ed, forming a box which can easily be handled by one man.'Ihe two free edges can be lfastened together by any convenient meanssuch as clips,

U-shaped nailsor adhesive tape. `It saves work and gives a very strongstructure if the board is so scored that two free edges meet on one ofthe anges of the member.

Instead of using rigid sheet material for constructing the inner casingwe may use iiexible brous material such as paper, pulp board, cardboardor the like in conjunction with reinforcement, e. g. wire netting. Ifdesired, the brous material may be previously treated with reproofingagents, e. g. ammonium phosphate, but this is not essential since thefibrous material is shielded from the effects of fire by the outercasing of gypsum, which produces a temperature gradient such that thetemperature onthe cold (inner) side is within the safe working limits ofthe fibrous material. Y

If we use board it is preferably a plaster board with at least one facecomposed of a liner which contains asbestos bre as part of thepapermaking fibres. In this way we obtain a reresisting junction betweenthe plaster and the core of the plasterboard, thus increasing thestability of the system when subjected to heat. Such a, fire-resistingjunction can also' be obtained by using a plaster-board with aperforated liner. On laying the plaster on such a plasterboard theplaster keys both to the liner and through the perforations to theplaster core so that even if the liner is subjected to suflicient heatto cause it to disintegrate or burn, the blocks will still be bonded tothe plaster core. As a bonding material to bond the blocks and to stickthem to the rigid backing, we prefer to use a calcium sulphate plaster,say anhydrite plaster or a hemihydrate plaster.

We prefer to employ blocks of void or cellular structure made fromplaster of the accelerated anhydrite type or of plaster of Paris. Suchblocks can be made from plaster mixes containing foam or suitable gasgenerating agents, e. g. as described in U. S. Patent No. 2,015,481.

Instead of using blocks we may surround the encased member with plastercast in situ. Thus the plasterboard next to the member to be protectedmay be used as shuttering in order to pour calcium sulphate protectingmaterial, using a temporary or' permanent outer shuttering, which outershuttering can be of the same type of board'. If it is to be permanent,lwe prefer to employ the board with at least one liner containing:asbestos, this liner contacting with the inner, poured calcium sulphatematerial. If we wish we can employ this type of board as a further outercasing for the system in which blocks arexused instead ofpouredmaterialas the main nre-resisting element. v 'The' accompanying drawingillustrates two applications of the invention. In these cases thestructure to be protected is a steel column. Both Figures 1 and 2represent a cross-section through the column and the protectivestructure erected round it. Figure 3 is a graph illustrating thefireproong testr of Example l. Figure 4 is a broken perspective sketchof a preferred form of the inner casing.

Referring to Figure 1, I is the steel column to to be protected; 2`represents an inner casing of inch gypsum plasterboard of the type knownas baseboard, while 3 represents an outer casing of 21,42 inch groovedcellular anhydrite blocks which are bonded to the baseboard and to eachother with anhydrite plaster.

Referring to Figure 2, II represents the column. to be protected, I 2 a3A inch mesh wire netting strapped in position with 1%, inch steelribbon, I3 a .0.03 inch wallboardl liner wrapped outside the netting I2,and I 4 another layer of 3A inch wire netting strapped in position asbefore. blocks which are bonded together and to the wallboard liner.

Referring to Figure 4, 2l is a sheet of plasterboard consisting of acore of calcium sulphate plaster between two liners 22 and 23. Thisboard has been scored along the vertical lines 24, thus cutting theliner 22 completely and`cuttingI to a certain extent into the core. Theboard was then bent along the lines 24 so that the core fractured whilethe liner 213 was undamaged. This sheet could then be wrappedv into theform shown in the drawing, in which shape it is intended to surround themember to be protected.

It can be held in this position by steel strappingv .5

or other means and then the blocks bonded to the outside.

Figure 3 will be further described in Example 1. The following examplesillustrate but d not limit the-invention.

I5 represents the outer casing of 2 inch Example 1 The object in thisexample was to protect a girder and the assembly was as illustrated inFigure 1. inch gypsum plasterboard 2 of the type known as baseboard wasstrapped to the steel column I to be protected by means of inch steelribbon spaced at intervals of 11/2 feet. The girder was a 5 foot X 6inch X 5 inch standard I section. 21/2 inch grooved cellular anhydriteblocks 3 were stuck to the baseboard with anhydrite plaster and werereinforced at the horizontal joints with steel 1 inch X l@ inch web Tsections. Anhydrite plaster was used for bonding the block wall and thejoints were scrimmed with 4 inch Hessian scrim. Subsequently, the blockswere slurried, i. e. thinly coated, with a wet cement wash, and afinishing coat of anhydrite plaster was applied to them.

The encasement was erected one week before testing and the skimming ornishing coat of plaster was applied 4 days before testing. The unit asdescribed wasI erected in a suitably designed test chamber which wastted with two burners placed 3 feet in front of the unit. Using cokeoven gas as fuel, a temperature of 950-1000 C. could be maintained atthe face of the system. l

Sufficient thermocouples were inserted to enable the temperaturedistribution in the system to be accurately ascertained, and they wereplaced in the following positions: one set on the external faces of theencasement in immediate contact with the flames, another on the internalfaces of the baseboard, and another on the steel girder itself. Thetemperature could then be read olf from suitable indicators.

The duration of the test was for 4 hours and observations of thetemperature rise and of the behaviour ofthe unit were made. Thetemperature curvesare given in Figure 3.

During the 4 hours of the test, the average temperature of the girderdid not rise above C., although the temperature of the encasement facewas about 1000 C.

' To ensure the safety of steelwork it is necessary that it should bekept down to a temperature below 500 C. It will be seen that thisparticular encasement therefore gives complete protection for at least 4hours and therefore gives remarkably good fire resistance.

In contrast with this, a 2 inch thick concrete slab attained atemperature of C. on the back face after 30 minutes.y

Eample 2 This example illustrates the e'lectiveness of the assemblyillustrated in Figure 2.

An encasement was prepared as follows: inch mesh light wire netting I2was wrapped once round the column II (5 foot x 6 inch X 5 inch) and thiswas strapped with inch steel ribbon at 18 inch intervals. On top of thiswas wound a single layer of wallboard liner I3, followed by'a secondlayer of the wire netting I4 which Was strapped With steel ribbon. Awall of 2 inch cellular anhydrite block I5 was then built against thisconstruction using a lire resistant anhydrite adhesive to secure theblocks to one another and to the liner and netting. Subsequently, thisWas nished with a single coat of anhydrite plaster.

The encasement had not failed after 2%. hours, testing by the standardtest, and at that time the average temperature of the girder was 112 C.

Instead of cellular anhydrite blocks, compact anhydrite blocks, cellularor compact hemihydrate blocks may be used. Cellular anhydrite blocksreinforced on both large faces with paper stuck with calcium caseinate(of the type described under U. S. application No. 171,788) may be usedwith advantage.

We prefer to use blocks having a thickness of at least 3A inch, whichthickness will maintain the temperature of the cold face at below thedehydration temperature of the plaster for several hours for anytemperature of the hot face normally met with during a re.

This invention is of course not limited merely to building protectivestructures round girders but could equally well be applied to buildingfireproof Walls and the like.

This invention is a valuable advance in the art as the blocks are keyedover a Whole surface to a strong backing, so that even if the outercasing cracks pieces will not fall away. This means that no ame canreach the girder until the blocks have almost entirely disintegrated, sothat the protective system has a longer life than that of any knownsystem of similar cost.

As many apparently Widely dilerent embodin ments of this invention maybe made without departing from the spirit and scope thereof, it is to beunderstood that the invention is not limited to the specific embodimentsthereof except as defined in the appended claims;

We claim:

1. A rire-resisting casing for beams and co1- umns in structures, whichcomprises an outer casing of set calcium sulphate plaster of at least 3Ainch thickness firmly bonded over the whole of its inner surface to abacking layer of strong non-Woven fibrous sheet material, said backinglayer forming a box-like inner structure with rectangular sidesenclosing the member to be protected and spaced from portions of saidmember to provide longitudinal air spaces adjacent thereto, thecross-sectional dimensions of said structure being substantially justsuflcient to accommodate said member.

2. A fire-resisting casing according to claim l, in which the innerstructure comprises plasterboard having an outer liner which containsasbestos fiber as part of the paper-making bers.

3. A fire-resisting casing according to claim 1, in which the innerstructure comprises plasterboard having an outer liner which isperforated so as to expose the plaster core at a number of places andthereby furnish a key for subsequently-applied plaster.

4. A fire-resisting casing according to claim 1, in which said box-likestructure is composed of a plurality of rectangular strips ofplasterboard having a plastic core rmly bonded to inner and outer linersof non-Woven fibrous sheet material, said strips being united by saidinner liner, and said outer liner and core being fractured at thecorners of said structure.

5. A nre-resisting casing according to claim l, in which said box-likestructure consists of flexible material belonging to the classconsisting of paper, pulp board and cardboard, in conjunction With metalreticulated reinforcement.

6. A fire-resisting casing according to claim 1, in Which the outerstructure consists of blocks bonded together and to the inner casing.

7. A fire-resisting casing according to claim 1, in which the outercasing consists of blocks bonded together and to the inner casing with acalcium sulphate plaster.

8. A re-resisting casing according to claim 1, in which the outer casingconsists of plaster cast in situ.

VICTOR LEFEBURE. ARTHUR HENRY DOUGLAS.

